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From yh...@apache.org
Subject spark git commit: [SPARK-8638] [SQL] Window Function Performance Improvements
Date Sun, 19 Jul 2015 06:44:42 GMT
Repository: spark
Updated Branches:
  refs/heads/master 04c1b49f5 -> a9a0d0ceb


[SPARK-8638] [SQL] Window Function Performance Improvements

## Description
Performance improvements for Spark Window functions. This PR will also serve as the basis for moving away from Hive UDAFs to Spark UDAFs. See JIRA tickets SPARK-8638 and SPARK-7712 for more information.

## Improvements
* Much better performance (10x) in running cases (e.g. BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) and UNBOUDED FOLLOWING cases. The current implementation in spark uses a sliding window approach in these cases. This means that an aggregate is maintained for every row, so space usage is N (N being the number of rows). This also means that all these aggregates all need to be updated separately, this takes N*(N-1)/2 updates. The running case differs from the Sliding case because we are only adding data to an aggregate function (no reset is required), we only need to maintain one aggregate (like in the UNBOUNDED PRECEDING AND UNBOUNDED case), update the aggregate for each row, and get the aggregate value after each update. This is what the new implementation does. This approach only uses 1 buffer, and only requires N updates; I am currently working on data with window sizes of 500-1000 doing running sums and this saves a lot of time. The CURRENT ROW AND UNBOUNDED FOLLOWING case also us
 es this approach and the fact that aggregate operations are communitative, there is one twist though it will process the input buffer in reverse.
* Fewer comparisons in the sliding case. The current implementation determines frame boundaries for every input row. The new implementation makes more use of the fact that the window is sorted, maintains the boundaries, and only moves them when the current row order changes. This is a minor improvement.
* A single Window node is able to process all types of Frames for the same Partitioning/Ordering. This saves a little time/memory spent buffering and managing partitions. This will be enabled in a follow-up PR.
* A lot of the staging code is moved from the execution phase to the initialization phase. Minor performance improvement, and improves readability of the execution code.

## Benchmarking
I have done a small benchmark using [on time performance](http://www.transtats.bts.gov) data of the month april. I have used the origin as a partioning key, as a result there is quite some variation in window sizes. The code for the benchmark can be found in the JIRA ticket. These are the results per Frame type:

Frame | Master | SPARK-8638
----- | ------ | ----------
Entire Frame | 2 s | 1 s
Sliding | 18 s | 1 s
Growing | 14 s | 0.9 s
Shrinking | 13 s | 1 s

Author: Herman van Hovell <hvanhovell@questtec.nl>

Closes #7057 from hvanhovell/SPARK-8638 and squashes the following commits:

3bfdc49 [Herman van Hovell] Fixed Perfomance Regression for Shrinking Window Frames (+Rebase)
2eb3b33 [Herman van Hovell] Corrected reverse range frame processing.
2cd2d5b [Herman van Hovell] Corrected reverse range frame processing.
b0654d7 [Herman van Hovell] Tests for exotic frame specifications.
e75b76e [Herman van Hovell] More docs, added support for reverse sliding range frames, and some reorganization of code.
1fdb558 [Herman van Hovell] Changed Data In HiveDataFrameWindowSuite.
ac2f682 [Herman van Hovell] Added a few more comments.
1938312 [Herman van Hovell] Added Documentation to the createBoundOrdering methods.
bb020e6 [Herman van Hovell] Major overhaul of Window operator.


Project: http://git-wip-us.apache.org/repos/asf/spark/repo
Commit: http://git-wip-us.apache.org/repos/asf/spark/commit/a9a0d0ce
Tree: http://git-wip-us.apache.org/repos/asf/spark/tree/a9a0d0ce
Diff: http://git-wip-us.apache.org/repos/asf/spark/diff/a9a0d0ce

Branch: refs/heads/master
Commit: a9a0d0cebf8ab3c539723488e5945794ebfd6104
Parents: 04c1b49
Author: Herman van Hovell <hvanhovell@questtec.nl>
Authored: Sat Jul 18 23:44:38 2015 -0700
Committer: Yin Huai <yhuai@databricks.com>
Committed: Sat Jul 18 23:44:38 2015 -0700

----------------------------------------------------------------------
 .../expressions/windowExpressions.scala         |   12 +
 .../org/apache/spark/sql/execution/Window.scala | 1072 +++++++++++-------
 .../sql/hive/HiveDataFrameWindowSuite.scala     |    6 +-
 .../spark/sql/hive/execution/WindowSuite.scala  |   79 ++
 4 files changed, 765 insertions(+), 404 deletions(-)
----------------------------------------------------------------------


http://git-wip-us.apache.org/repos/asf/spark/blob/a9a0d0ce/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala
----------------------------------------------------------------------
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala
index 50bbfd6..09ec0e3 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala
@@ -316,3 +316,15 @@ case class WindowExpression(
 
   override def toString: String = s"$windowFunction $windowSpec"
 }
+
+/**
+ * Extractor for making working with frame boundaries easier.
+ */
+object FrameBoundaryExtractor {
+  def unapply(boundary: FrameBoundary): Option[Int] = boundary match {
+    case CurrentRow => Some(0)
+    case ValuePreceding(offset) => Some(-offset)
+    case ValueFollowing(offset) => Some(offset)
+    case _ => None
+  }
+}

http://git-wip-us.apache.org/repos/asf/spark/blob/a9a0d0ce/sql/core/src/main/scala/org/apache/spark/sql/execution/Window.scala
----------------------------------------------------------------------
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/Window.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/Window.scala
index 6e127e5..a054f52 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/Window.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/Window.scala
@@ -19,18 +19,64 @@ package org.apache.spark.sql.execution
 
 import java.util
 
-import org.apache.spark.rdd.RDD
+import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.expressions._
-import org.apache.spark.sql.catalyst.plans.physical.{AllTuples, ClusteredDistribution, Distribution, Partitioning}
+import org.apache.spark.sql.catalyst.plans.physical._
+import org.apache.spark.sql.types.IntegerType
+import org.apache.spark.rdd.RDD
 import org.apache.spark.util.collection.CompactBuffer
+import scala.collection.mutable
 
 /**
  * :: DeveloperApi ::
- * For every row, evaluates `windowExpression` containing Window Functions and attaches
- * the results with other regular expressions (presented by `projectList`).
- * Evert operator handles a single Window Specification, `windowSpec`.
+ * This class calculates and outputs (windowed) aggregates over the rows in a single (sorted)
+ * partition. The aggregates are calculated for each row in the group. Special processing
+ * instructions, frames, are used to calculate these aggregates. Frames are processed in the order
+ * specified in the window specification (the ORDER BY ... clause). There are four different frame
+ * types:
+ * - Entire partition: The frame is the entire partition, i.e.
+ *   UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING. For this case, window function will take all
+ *   rows as inputs and be evaluated once.
+ * - Growing frame: We only add new rows into the frame, i.e. UNBOUNDED PRECEDING AND ....
+ *   Every time we move to a new row to process, we add some rows to the frame. We do not remove
+ *   rows from this frame.
+ * - Shrinking frame: We only remove rows from the frame, i.e. ... AND UNBOUNDED FOLLOWING.
+ *   Every time we move to a new row to process, we remove some rows from the frame. We do not add
+ *   rows to this frame.
+ * - Moving frame: Every time we move to a new row to process, we remove some rows from the frame
+ *   and we add some rows to the frame. Examples are:
+ *     1 PRECEDING AND CURRENT ROW and 1 FOLLOWING AND 2 FOLLOWING.
+ *
+ * Different frame boundaries can be used in Growing, Shrinking and Moving frames. A frame
+ * boundary can be either Row or Range based:
+ * - Row Based: A row based boundary is based on the position of the row within the partition.
+ *   An offset indicates the number of rows above or below the current row, the frame for the
+ *   current row starts or ends. For instance, given a row based sliding frame with a lower bound
+ *   offset of -1 and a upper bound offset of +2. The frame for row with index 5 would range from
+ *   index 4 to index 6.
+ * - Range based: A range based boundary is based on the actual value of the ORDER BY
+ *   expression(s). An offset is used to alter the value of the ORDER BY expression, for
+ *   instance if the current order by expression has a value of 10 and the lower bound offset
+ *   is -3, the resulting lower bound for the current row will be 10 - 3 = 7. This however puts a
+ *   number of constraints on the ORDER BY expressions: there can be only one expression and this
+ *   expression must have a numerical data type. An exception can be made when the offset is 0,
+ *   because no value modification is needed, in this case multiple and non-numeric ORDER BY
+ *   expression are allowed.
+ *
+ * This is quite an expensive operator because every row for a single group must be in the same
+ * partition and partitions must be sorted according to the grouping and sort order. The operator
+ * requires the planner to take care of the partitioning and sorting.
+ *
+ * The operator is semi-blocking. The window functions and aggregates are calculated one group at
+ * a time, the result will only be made available after the processing for the entire group has
+ * finished. The operator is able to process different frame configurations at the same time. This
+ * is done by delegating the actual frame processing (i.e. calculation of the window functions) to
+ * specialized classes, see [[WindowFunctionFrame]], which take care of their own frame type:
+ * Entire Partition, Sliding, Growing & Shrinking. Boundary evaluation is also delegated to a pair
+ * of specialized classes: [[RowBoundOrdering]] & [[RangeBoundOrdering]].
  */
+@DeveloperApi
 case class Window(
     projectList: Seq[Attribute],
     windowExpression: Seq[NamedExpression],
@@ -38,443 +84,667 @@ case class Window(
     child: SparkPlan)
   extends UnaryNode {
 
-  override def output: Seq[Attribute] =
-    (projectList ++ windowExpression).map(_.toAttribute)
+  override def output: Seq[Attribute] = projectList ++ windowExpression.map(_.toAttribute)
 
-  override def requiredChildDistribution: Seq[Distribution] =
+  override def requiredChildDistribution: Seq[Distribution] = {
     if (windowSpec.partitionSpec.isEmpty) {
-      // This operator will be very expensive.
+      // Only show warning when the number of bytes is larger than 100 MB?
+      logWarning("No Partition Defined for Window operation! Moving all data to a single "
+        + "partition, this can cause serious performance degradation.")
       AllTuples :: Nil
-    } else {
-      ClusteredDistribution(windowSpec.partitionSpec) :: Nil
-    }
-
-  // Since window functions are adding columns to the input rows, the child's outputPartitioning
-  // is preserved.
-  override def outputPartitioning: Partitioning = child.outputPartitioning
-
-  override def requiredChildOrdering: Seq[Seq[SortOrder]] = {
-    // The required child ordering has two parts.
-    // The first part is the expressions in the partition specification.
-    // We add these expressions to the required ordering to make sure input rows are grouped
-    // based on the partition specification. So, we only need to process a single partition
-    // at a time.
-    // The second part is the expressions specified in the ORDER BY cluase.
-    // Basically, we first use sort to group rows based on partition specifications and then sort
-    // Rows in a group based on the order specification.
-    (windowSpec.partitionSpec.map(SortOrder(_, Ascending)) ++ windowSpec.orderSpec) :: Nil
+    } else ClusteredDistribution(windowSpec.partitionSpec) :: Nil
   }
 
-  // Since window functions basically add columns to input rows, this operator
-  // will not change the ordering of input rows.
+  override def requiredChildOrdering: Seq[Seq[SortOrder]] =
+    Seq(windowSpec.partitionSpec.map(SortOrder(_, Ascending)) ++ windowSpec.orderSpec)
+
   override def outputOrdering: Seq[SortOrder] = child.outputOrdering
 
-  case class ComputedWindow(
-    unbound: WindowExpression,
-    windowFunction: WindowFunction,
-    resultAttribute: AttributeReference)
-
-  // A list of window functions that need to be computed for each group.
-  private[this] val computedWindowExpressions = windowExpression.flatMap { window =>
-    window.collect {
-      case w: WindowExpression =>
-        ComputedWindow(
-          w,
-          BindReferences.bindReference(w.windowFunction, child.output),
-          AttributeReference(s"windowResult:$w", w.dataType, w.nullable)())
+  /**
+   * Create a bound ordering object for a given frame type and offset. A bound ordering object is
+   * used to determine which input row lies within the frame boundaries of an output row.
+   *
+   * This method uses Code Generation. It can only be used on the executor side.
+   *
+   * @param frameType to evaluate. This can either be Row or Range based.
+   * @param offset with respect to the row.
+   * @return a bound ordering object.
+   */
+  private[this] def createBoundOrdering(frameType: FrameType, offset: Int): BoundOrdering = {
+    frameType match {
+      case RangeFrame =>
+        val (exprs, current, bound) = if (offset == 0) {
+          // Use the entire order expression when the offset is 0.
+          val exprs = windowSpec.orderSpec.map(_.child)
+          val projection = newMutableProjection(exprs, child.output)
+          (windowSpec.orderSpec, projection(), projection())
+        }
+        else if (windowSpec.orderSpec.size == 1) {
+          // Use only the first order expression when the offset is non-null.
+          val sortExpr = windowSpec.orderSpec.head
+          val expr = sortExpr.child
+          // Create the projection which returns the current 'value'.
+          val current = newMutableProjection(expr :: Nil, child.output)()
+          // Flip the sign of the offset when processing the order is descending
+          val boundOffset = if (sortExpr.direction == Descending) -offset
+          else offset
+          // Create the projection which returns the current 'value' modified by adding the offset.
+          val boundExpr = Add(expr, Cast(Literal.create(boundOffset, IntegerType), expr.dataType))
+          val bound = newMutableProjection(boundExpr :: Nil, child.output)()
+          (sortExpr :: Nil, current, bound)
+        }
+        else {
+          sys.error("Non-Zero range offsets are not supported for windows " +
+            "with multiple order expressions.")
+        }
+        // Construct the ordering. This is used to compare the result of current value projection
+        // to the result of bound value projection. This is done manually because we want to use
+        // Code Generation (if it is enabled).
+        val (sortExprs, schema) = exprs.map { case e =>
+          val ref = AttributeReference("ordExpr", e.dataType, e.nullable)()
+          (SortOrder(ref, e.direction), ref)
+        }.unzip
+        val ordering = newOrdering(sortExprs, schema)
+        RangeBoundOrdering(ordering, current, bound)
+      case RowFrame => RowBoundOrdering(offset)
     }
-  }.toArray
+  }
 
-  private[this] val windowFrame =
-    windowSpec.frameSpecification.asInstanceOf[SpecifiedWindowFrame]
+  /**
+   * Create a frame processor.
+   *
+   * This method uses Code Generation. It can only be used on the executor side.
+   *
+   * @param frame boundaries.
+   * @param functions to process in the frame.
+   * @param ordinal at which the processor starts writing to the output.
+   * @return a frame processor.
+   */
+  private[this] def createFrameProcessor(
+      frame: WindowFrame,
+      functions: Array[WindowFunction],
+      ordinal: Int): WindowFunctionFrame = frame match {
+    // Growing Frame.
+    case SpecifiedWindowFrame(frameType, UnboundedPreceding, FrameBoundaryExtractor(high)) =>
+      val uBoundOrdering = createBoundOrdering(frameType, high)
+      new UnboundedPrecedingWindowFunctionFrame(ordinal, functions, uBoundOrdering)
+
+    // Shrinking Frame.
+    case SpecifiedWindowFrame(frameType, FrameBoundaryExtractor(low), UnboundedFollowing) =>
+      val lBoundOrdering = createBoundOrdering(frameType, low)
+      new UnboundedFollowingWindowFunctionFrame(ordinal, functions, lBoundOrdering)
+
+    // Moving Frame.
+    case SpecifiedWindowFrame(frameType,
+        FrameBoundaryExtractor(low), FrameBoundaryExtractor(high)) =>
+      val lBoundOrdering = createBoundOrdering(frameType, low)
+      val uBoundOrdering = createBoundOrdering(frameType, high)
+      new SlidingWindowFunctionFrame(ordinal, functions, lBoundOrdering, uBoundOrdering)
+
+    // Entire Partition Frame.
+    case SpecifiedWindowFrame(_, UnboundedPreceding, UnboundedFollowing) =>
+      new UnboundedWindowFunctionFrame(ordinal, functions)
+
+    // Error
+    case fr =>
+      sys.error(s"Unsupported Frame $fr for functions: $functions")
+  }
 
-  // Create window functions.
-  private[this] def windowFunctions(): Array[WindowFunction] = {
-    val functions = new Array[WindowFunction](computedWindowExpressions.length)
-    var i = 0
-    while (i < computedWindowExpressions.length) {
-      functions(i) = computedWindowExpressions(i).windowFunction.newInstance()
-      functions(i).init()
-      i += 1
+  /**
+   * Create the resulting projection.
+   *
+   * This method uses Code Generation. It can only be used on the executor side.
+   *
+   * @param expressions unbound ordered function expressions.
+   * @return the final resulting projection.
+   */
+  private[this] def createResultProjection(
+      expressions: Seq[Expression]): MutableProjection = {
+    val unboundToAttr = expressions.map {
+      e => (e, AttributeReference("windowResult", e.dataType, e.nullable)())
     }
-    functions
+    val unboundToAttrMap = unboundToAttr.toMap
+    val patchedWindowExpression = windowExpression.map(_.transform(unboundToAttrMap))
+    newMutableProjection(
+      projectList ++ patchedWindowExpression,
+      child.output ++ unboundToAttr.map(_._2))()
   }
 
-  // The schema of the result of all window function evaluations
-  private[this] val computedSchema = computedWindowExpressions.map(_.resultAttribute)
-
-  private[this] val computedResultMap =
-    computedWindowExpressions.map { w => w.unbound -> w.resultAttribute }.toMap
+  protected override def doExecute(): RDD[InternalRow] = {
+    // Prepare processing.
+    // Group the window expression by their processing frame.
+    val windowExprs = windowExpression.flatMap {
+      _.collect {
+        case e: WindowExpression => e
+      }
+    }
 
-  private[this] val windowExpressionResult = windowExpression.map { window =>
-    window.transform {
-      case w: WindowExpression if computedResultMap.contains(w) => computedResultMap(w)
+    // Create Frame processor factories and order the unbound window expressions by the frame they
+    // are processed in; this is the order in which their results will be written to window
+    // function result buffer.
+    val framedWindowExprs = windowExprs.groupBy(_.windowSpec.frameSpecification)
+    val factories = Array.ofDim[() => WindowFunctionFrame](framedWindowExprs.size)
+    val unboundExpressions = mutable.Buffer.empty[Expression]
+    framedWindowExprs.zipWithIndex.foreach {
+      case ((frame, unboundFrameExpressions), index) =>
+        // Track the ordinal.
+        val ordinal = unboundExpressions.size
+
+        // Track the unbound expressions
+        unboundExpressions ++= unboundFrameExpressions
+
+        // Bind the expressions.
+        val functions = unboundFrameExpressions.map { e =>
+          BindReferences.bindReference(e.windowFunction, child.output)
+        }.toArray
+
+        // Create the frame processor factory.
+        factories(index) = () => createFrameProcessor(frame, functions, ordinal)
     }
-  }
 
-  protected override def doExecute(): RDD[InternalRow] = {
-    child.execute().mapPartitions { iter =>
+    // Start processing.
+    child.execute().mapPartitions { stream =>
       new Iterator[InternalRow] {
 
-        // Although input rows are grouped based on windowSpec.partitionSpec, we need to
-        // know when we have a new partition.
-        // This is to manually construct an ordering that can be used to compare rows.
-        // TODO: We may want to have a newOrdering that takes BoundReferences.
-        // So, we can take advantave of code gen.
-        private val partitionOrdering: Ordering[InternalRow] =
-          RowOrdering.forSchema(windowSpec.partitionSpec.map(_.dataType))
-
-        // This is used to project expressions for the partition specification.
-        protected val partitionGenerator =
-          newMutableProjection(windowSpec.partitionSpec, child.output)()
-
-        // This is ued to project expressions for the order specification.
-        protected val rowOrderGenerator =
-          newMutableProjection(windowSpec.orderSpec.map(_.child), child.output)()
-
-        // The position of next output row in the inputRowBuffer.
-        var rowPosition: Int = 0
-        // The number of buffered rows in the inputRowBuffer (the size of the current partition).
-        var partitionSize: Int = 0
-        // The buffer used to buffer rows in a partition.
-        var inputRowBuffer: CompactBuffer[InternalRow] = _
-        // The partition key of the current partition.
-        var currentPartitionKey: InternalRow = _
-        // The partition key of next partition.
-        var nextPartitionKey: InternalRow = _
-        // The first row of next partition.
-        var firstRowInNextPartition: InternalRow = _
-        // Indicates if this partition is the last one in the iter.
-        var lastPartition: Boolean = false
-
-        def createBoundaryEvaluator(): () => Unit = {
-          def findPhysicalBoundary(
-              boundary: FrameBoundary): () => Int = boundary match {
-            case UnboundedPreceding => () => 0
-            case UnboundedFollowing => () => partitionSize - 1
-            case CurrentRow => () => rowPosition
-            case ValuePreceding(value) =>
-              () =>
-                val newPosition = rowPosition - value
-                if (newPosition > 0) newPosition else 0
-            case ValueFollowing(value) =>
-              () =>
-                val newPosition = rowPosition + value
-                if (newPosition < partitionSize) newPosition else partitionSize - 1
+        // Get all relevant projections.
+        val result = createResultProjection(unboundExpressions)
+        val grouping = newProjection(windowSpec.partitionSpec, child.output)
+
+        // Manage the stream and the grouping.
+        var nextRow: InternalRow = EmptyRow
+        var nextGroup: InternalRow = EmptyRow
+        var nextRowAvailable: Boolean = false
+        private[this] def fetchNextRow() {
+          nextRowAvailable = stream.hasNext
+          if (nextRowAvailable) {
+            nextRow = stream.next()
+            nextGroup = grouping(nextRow)
+          } else {
+            nextRow = EmptyRow
+            nextGroup = EmptyRow
           }
-
-          def findLogicalBoundary(
-              boundary: FrameBoundary,
-              searchDirection: Int,
-              evaluator: Expression,
-              joinedRow: JoinedRow): () => Int = boundary match {
-            case UnboundedPreceding => () => 0
-            case UnboundedFollowing => () => partitionSize - 1
-            case other =>
-              () => {
-                // CurrentRow, ValuePreceding, or ValueFollowing.
-                var newPosition = rowPosition + searchDirection
-                var stopSearch = false
-                // rowOrderGenerator is a mutable projection.
-                // We need to make a copy of the returned by rowOrderGenerator since we will
-                // compare searched row with this currentOrderByValue.
-                val currentOrderByValue = rowOrderGenerator(inputRowBuffer(rowPosition)).copy()
-                while (newPosition >= 0 && newPosition < partitionSize && !stopSearch) {
-                  val r = rowOrderGenerator(inputRowBuffer(newPosition))
-                  stopSearch =
-                    !(evaluator.eval(joinedRow(currentOrderByValue, r)).asInstanceOf[Boolean])
-                  if (!stopSearch) {
-                    newPosition += searchDirection
-                  }
-                }
-                newPosition -= searchDirection
-
-                if (newPosition < 0) {
-                  0
-                } else if (newPosition >= partitionSize) {
-                  partitionSize - 1
-                } else {
-                  newPosition
-                }
-              }
+        }
+        fetchNextRow()
+
+        // Manage the current partition.
+        var rows: CompactBuffer[InternalRow] = _
+        val frames: Array[WindowFunctionFrame] = factories.map(_())
+        val numFrames = frames.length
+        private[this] def fetchNextPartition() {
+          // Collect all the rows in the current partition.
+          val currentGroup = nextGroup
+          rows = new CompactBuffer
+          while (nextRowAvailable && nextGroup == currentGroup) {
+            rows += nextRow.copy()
+            fetchNextRow()
           }
 
-          windowFrame.frameType match {
-            case RowFrame =>
-              val findStart = findPhysicalBoundary(windowFrame.frameStart)
-              val findEnd = findPhysicalBoundary(windowFrame.frameEnd)
-              () => {
-                frameStart = findStart()
-                frameEnd = findEnd()
-              }
-            case RangeFrame =>
-              val joinedRowForBoundaryEvaluation: JoinedRow = new JoinedRow()
-              val orderByExpr = windowSpec.orderSpec.head
-              val currentRowExpr =
-                BoundReference(0, orderByExpr.dataType, orderByExpr.nullable)
-              val examedRowExpr =
-                BoundReference(1, orderByExpr.dataType, orderByExpr.nullable)
-              val differenceExpr = Abs(Subtract(currentRowExpr, examedRowExpr))
-
-              val frameStartEvaluator = windowFrame.frameStart match {
-                case CurrentRow => EqualTo(currentRowExpr, examedRowExpr)
-                case ValuePreceding(value) =>
-                  LessThanOrEqual(differenceExpr, Cast(Literal(value), orderByExpr.dataType))
-                case ValueFollowing(value) =>
-                  GreaterThanOrEqual(differenceExpr, Cast(Literal(value), orderByExpr.dataType))
-                case o => Literal(true) // This is just a dummy expression, we will not use it.
-              }
-
-              val frameEndEvaluator = windowFrame.frameEnd match {
-                case CurrentRow => EqualTo(currentRowExpr, examedRowExpr)
-                case ValuePreceding(value) =>
-                  GreaterThanOrEqual(differenceExpr, Cast(Literal(value), orderByExpr.dataType))
-                case ValueFollowing(value) =>
-                  LessThanOrEqual(differenceExpr, Cast(Literal(value), orderByExpr.dataType))
-                case o => Literal(true) // This is just a dummy expression, we will not use it.
-              }
-
-              val findStart =
-                findLogicalBoundary(
-                  boundary = windowFrame.frameStart,
-                  searchDirection = -1,
-                  evaluator = frameStartEvaluator,
-                  joinedRow = joinedRowForBoundaryEvaluation)
-              val findEnd =
-                findLogicalBoundary(
-                  boundary = windowFrame.frameEnd,
-                  searchDirection = 1,
-                  evaluator = frameEndEvaluator,
-                  joinedRow = joinedRowForBoundaryEvaluation)
-              () => {
-                frameStart = findStart()
-                frameEnd = findEnd()
-              }
+          // Setup the frames.
+          var i = 0
+          while (i < numFrames) {
+            frames(i).prepare(rows)
+            i += 1
           }
+
+          // Setup iteration
+          rowIndex = 0
+          rowsSize = rows.size
         }
 
-        val boundaryEvaluator = createBoundaryEvaluator()
-        // Indicates if we the specified window frame requires us to maintain a sliding frame
-        // (e.g. RANGES BETWEEN 1 PRECEDING AND CURRENT ROW) or the window frame
-        // is the entire partition (e.g. ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING).
-        val requireUpdateFrame: Boolean = {
-          def requireUpdateBoundary(boundary: FrameBoundary): Boolean = boundary match {
-            case UnboundedPreceding => false
-            case UnboundedFollowing => false
-            case _ => true
-          }
+        // Iteration
+        var rowIndex = 0
+        var rowsSize = 0
+        override final def hasNext: Boolean = rowIndex < rowsSize || nextRowAvailable
 
-          requireUpdateBoundary(windowFrame.frameStart) ||
-            requireUpdateBoundary(windowFrame.frameEnd)
-        }
-        // The start position of the current frame in the partition.
-        var frameStart: Int = 0
-        // The end position of the current frame in the partition.
-        var frameEnd: Int = -1
-        // Window functions.
-        val functions: Array[WindowFunction] = windowFunctions()
-        // Buffers used to store input parameters for window functions. Because we may need to
-        // maintain a sliding frame, we use this buffer to avoid evaluate the parameters from
-        // the same row multiple times.
-        val windowFunctionParameterBuffers: Array[util.LinkedList[AnyRef]] =
-          functions.map(_ => new util.LinkedList[AnyRef]())
-
-        // The projection used to generate the final result rows of this operator.
-        private[this] val resultProjection =
-          newMutableProjection(
-            projectList ++ windowExpressionResult,
-            projectList ++ computedSchema)()
-
-        // The row used to hold results of window functions.
-        private[this] val windowExpressionResultRow =
-          new GenericMutableRow(computedSchema.length)
-
-        private[this] val joinedRow = new JoinedRow6
-
-        // Initialize this iterator.
-        initialize()
-
-        private def initialize(): Unit = {
-          if (iter.hasNext) {
-            val currentRow = iter.next().copy()
-            // partitionGenerator is a mutable projection. Since we need to track nextPartitionKey,
-            // we are making a copy of the returned partitionKey at here.
-            nextPartitionKey = partitionGenerator(currentRow).copy()
-            firstRowInNextPartition = currentRow
+        val join = new JoinedRow6
+        val windowFunctionResult = new GenericMutableRow(unboundExpressions.size)
+        override final def next(): InternalRow = {
+          // Load the next partition if we need to.
+          if (rowIndex >= rowsSize && nextRowAvailable) {
             fetchNextPartition()
-          } else {
-            // The iter is an empty one. So, we set all of the following variables
-            // to make sure hasNext will return false.
-            lastPartition = true
-            rowPosition = 0
-            partitionSize = 0
           }
-        }
-
-        // Indicates if we will have new output row.
-        override final def hasNext: Boolean = {
-          !lastPartition || (rowPosition < partitionSize)
-        }
 
-        override final def next(): InternalRow = {
-          if (hasNext) {
-            if (rowPosition == partitionSize) {
-              // All rows of this buffer have been consumed.
-              // We will move to next partition.
-              fetchNextPartition()
-            }
-            // Get the input row for the current output row.
-            val inputRow = inputRowBuffer(rowPosition)
-            // Get all results of the window functions for this output row.
+          if (rowIndex < rowsSize) {
+            // Get the results for the window frames.
             var i = 0
-            while (i < functions.length) {
-              windowExpressionResultRow.update(i, functions(i).get(rowPosition))
+            while (i < numFrames) {
+              frames(i).write(windowFunctionResult)
               i += 1
             }
 
-            // Construct the output row.
-            val outputRow = resultProjection(joinedRow(inputRow, windowExpressionResultRow))
-            // We will move to the next one.
-            rowPosition += 1
-            if (requireUpdateFrame && rowPosition < partitionSize) {
-              // If we need to maintain a sliding frame and
-              // we will still work on this partition when next is called next time, do the update.
-              updateFrame()
-            }
+            // 'Merge' the input row with the window function result
+            join(rows(rowIndex), windowFunctionResult)
+            rowIndex += 1
 
-            // Return the output row.
-            outputRow
-          } else {
-            // no more result
-            throw new NoSuchElementException
-          }
+            // Return the projection.
+            result(join)
+          } else throw new NoSuchElementException
         }
+      }
+    }
+  }
+}
 
-        // Fetch the next partition.
-        private def fetchNextPartition(): Unit = {
-          // Create a new buffer for input rows.
-          inputRowBuffer = new CompactBuffer[InternalRow]()
-          // We already have the first row for this partition
-          // (recorded in firstRowInNextPartition). Add it back.
-          inputRowBuffer += firstRowInNextPartition
-          // Set the current partition key.
-          currentPartitionKey = nextPartitionKey
-          // Now, we will start to find all rows belonging to this partition.
-          // Create a variable to track if we see the next partition.
-          var findNextPartition = false
-          // The search will stop when we see the next partition or there is no
-          // input row left in the iter.
-          while (iter.hasNext && !findNextPartition) {
-            // Make a copy of the input row since we will put it in the buffer.
-            val currentRow = iter.next().copy()
-            // Get the partition key based on the partition specification.
-            // For the below compare method, we do not need to make a copy of partitionKey.
-            val partitionKey = partitionGenerator(currentRow)
-            // Check if the current row belongs the current input row.
-            val comparing = partitionOrdering.compare(currentPartitionKey, partitionKey)
-            if (comparing == 0) {
-              // This row is still in the current partition.
-              inputRowBuffer += currentRow
-            } else {
-              // The current input row is in a different partition.
-              findNextPartition = true
-              // partitionGenerator is a mutable projection.
-              // Since we need to track nextPartitionKey and we determine that it should be set
-              // as partitionKey, we are making a copy of the partitionKey at here.
-              nextPartitionKey = partitionKey.copy()
-              firstRowInNextPartition = currentRow
-            }
-          }
+/**
+ * Function for comparing boundary values.
+ */
+private[execution] abstract class BoundOrdering {
+  def compare(input: Seq[InternalRow], inputIndex: Int, outputIndex: Int): Int
+}
 
-          // We have not seen a new partition. It means that there is no new row in the
-          // iter. The current partition is the last partition of the iter.
-          if (!findNextPartition) {
-            lastPartition = true
-          }
+/**
+ * Compare the input index to the bound of the output index.
+ */
+private[execution] final case class RowBoundOrdering(offset: Int) extends BoundOrdering {
+  override def compare(input: Seq[InternalRow], inputIndex: Int, outputIndex: Int): Int =
+    inputIndex - (outputIndex + offset)
+}
 
-          // We have got all rows for the current partition.
-          // Set rowPosition to 0 (the next output row will be based on the first
-          // input row of this partition).
-          rowPosition = 0
-          // The size of this partition.
-          partitionSize = inputRowBuffer.size
-          // Reset all parameter buffers of window functions.
-          var i = 0
-          while (i < windowFunctionParameterBuffers.length) {
-            windowFunctionParameterBuffers(i).clear()
-            i += 1
-          }
-          frameStart = 0
-          frameEnd = -1
-          // Create the first window frame for this partition.
-          // If we do not need to maintain a sliding frame, this frame will
-          // have the entire partition.
-          updateFrame()
-        }
+/**
+ * Compare the value of the input index to the value bound of the output index.
+ */
+private[execution] final case class RangeBoundOrdering(
+    ordering: Ordering[InternalRow],
+    current: Projection,
+    bound: Projection) extends BoundOrdering {
+  override def compare(input: Seq[InternalRow], inputIndex: Int, outputIndex: Int): Int =
+    ordering.compare(current(input(inputIndex)), bound(input(outputIndex)))
+}
 
-        /** The function used to maintain the sliding frame. */
-        private def updateFrame(): Unit = {
-          // Based on the difference between the new frame and old frame,
-          // updates the buffers holding input parameters of window functions.
-          // We will start to prepare input parameters starting from the row
-          // indicated by offset in the input row buffer.
-          def updateWindowFunctionParameterBuffers(
-              numToRemove: Int,
-              numToAdd: Int,
-              offset: Int): Unit = {
-            // First, remove unneeded entries from the head of every buffer.
-            var i = 0
-            while (i < numToRemove) {
-              var j = 0
-              while (j < windowFunctionParameterBuffers.length) {
-                windowFunctionParameterBuffers(j).remove()
-                j += 1
-              }
-              i += 1
-            }
-            // Then, add needed entries to the tail of every buffer.
-            i = 0
-            while (i < numToAdd) {
-              var j = 0
-              while (j < windowFunctionParameterBuffers.length) {
-                // Ask the function to prepare the input parameters.
-                val parameters = functions(j).prepareInputParameters(inputRowBuffer(i + offset))
-                windowFunctionParameterBuffers(j).add(parameters)
-                j += 1
-              }
-              i += 1
-            }
-          }
+/**
+ * A window function calculates the results of a number of window functions for a window frame.
+ * Before use a frame must be prepared by passing it all the rows in the current partition. After
+ * preparation the update method can be called to fill the output rows.
+ *
+ * TODO How to improve performance? A few thoughts:
+ * - Window functions are expensive due to its distribution and ordering requirements.
+ * Unfortunately it is up to the Spark engine to solve this. Improvements in the form of project
+ * Tungsten are on the way.
+ * - The window frame processing bit can be improved though. But before we start doing that we
+ * need to see how much of the time and resources are spent on partitioning and ordering, and
+ * how much time and resources are spent processing the partitions. There are a couple ways to
+ * improve on the current situation:
+ * - Reduce memory footprint by performing streaming calculations. This can only be done when
+ * there are no Unbound/Unbounded Following calculations present.
+ * - Use Tungsten style memory usage.
+ * - Use code generation in general, and use the approach to aggregation taken in the
+ *   GeneratedAggregate class in specific.
+ *
+ * @param ordinal of the first column written by this frame.
+ * @param functions to calculate the row values with.
+ */
+private[execution] abstract class WindowFunctionFrame(
+    ordinal: Int,
+    functions: Array[WindowFunction]) {
+
+  // Make sure functions are initialized.
+  functions.foreach(_.init())
+
+  /** Number of columns the window function frame is managing */
+  val numColumns = functions.length
+
+  /**
+   * Create a fresh thread safe copy of the frame.
+   *
+   * @return the copied frame.
+   */
+  def copy: WindowFunctionFrame
+
+  /**
+   * Create new instances of the functions.
+   *
+   * @return an array containing copies of the current window functions.
+   */
+  protected final def copyFunctions: Array[WindowFunction] = functions.map(_.newInstance())
+
+  /**
+   * Prepare the frame for calculating the results for a partition.
+   *
+   * @param rows to calculate the frame results for.
+   */
+  def prepare(rows: CompactBuffer[InternalRow]): Unit
+
+  /**
+   * Write the result for the current row to the given target row.
+   *
+   * @param target row to write the result for the current row to.
+   */
+  def write(target: GenericMutableRow): Unit
+
+  /** Reset the current window functions. */
+  protected final def reset(): Unit = {
+    var i = 0
+    while (i < numColumns) {
+      functions(i).reset()
+      i += 1
+    }
+  }
 
-          // Record the current frame start point and end point before
-          // we update them.
-          val previousFrameStart = frameStart
-          val previousFrameEnd = frameEnd
-          boundaryEvaluator()
-          updateWindowFunctionParameterBuffers(
-            frameStart - previousFrameStart,
-            frameEnd - previousFrameEnd,
-            previousFrameEnd + 1)
-          // Evaluate the current frame.
-          evaluateCurrentFrame()
-        }
+  /** Prepare an input row for processing. */
+  protected final def prepare(input: InternalRow): Array[AnyRef] = {
+    val prepared = new Array[AnyRef](numColumns)
+    var i = 0
+    while (i < numColumns) {
+      prepared(i) = functions(i).prepareInputParameters(input)
+      i += 1
+    }
+    prepared
+  }
 
-        /** Evaluate the current window frame. */
-        private def evaluateCurrentFrame(): Unit = {
-          var i = 0
-          while (i < functions.length) {
-            // Reset the state of the window function.
-            functions(i).reset()
-            // Get all buffered input parameters based on rows of this window frame.
-            val inputParameters = windowFunctionParameterBuffers(i).toArray()
-            // Send these input parameters to the window function.
-            functions(i).batchUpdate(inputParameters)
-            // Ask the function to evaluate based on this window frame.
-            functions(i).evaluate()
-            i += 1
-          }
-        }
+  /** Evaluate a prepared buffer (iterator). */
+  protected final def evaluatePrepared(iterator: java.util.Iterator[Array[AnyRef]]): Unit = {
+    reset()
+    while (iterator.hasNext) {
+      val prepared = iterator.next()
+      var i = 0
+      while (i < numColumns) {
+        functions(i).update(prepared(i))
+        i += 1
       }
     }
+    evaluate()
   }
+
+  /** Evaluate a prepared buffer (array). */
+  protected final def evaluatePrepared(prepared: Array[Array[AnyRef]],
+      fromIndex: Int, toIndex: Int): Unit = {
+    var i = 0
+    while (i < numColumns) {
+      val function = functions(i)
+      function.reset()
+      var j = fromIndex
+      while (j < toIndex) {
+        function.update(prepared(j)(i))
+        j += 1
+      }
+      function.evaluate()
+      i += 1
+    }
+  }
+
+  /** Update an array of window functions. */
+  protected final def update(input: InternalRow): Unit = {
+    var i = 0
+    while (i < numColumns) {
+      val aggregate = functions(i)
+      val preparedInput = aggregate.prepareInputParameters(input)
+      aggregate.update(preparedInput)
+      i += 1
+    }
+  }
+
+  /** Evaluate the window functions. */
+  protected final def evaluate(): Unit = {
+    var i = 0
+    while (i < numColumns) {
+      functions(i).evaluate()
+      i += 1
+    }
+  }
+
+  /** Fill a target row with the current window function results. */
+  protected final def fill(target: GenericMutableRow, rowIndex: Int): Unit = {
+    var i = 0
+    while (i < numColumns) {
+      target.update(ordinal + i, functions(i).get(rowIndex))
+      i += 1
+    }
+  }
+}
+
+/**
+ * The sliding window frame calculates frames with the following SQL form:
+ * ... BETWEEN 1 PRECEDING AND 1 FOLLOWING
+ *
+ * @param ordinal of the first column written by this frame.
+ * @param functions to calculate the row values with.
+ * @param lbound comparator used to identify the lower bound of an output row.
+ * @param ubound comparator used to identify the upper bound of an output row.
+ */
+private[execution] final class SlidingWindowFunctionFrame(
+    ordinal: Int,
+    functions: Array[WindowFunction],
+    lbound: BoundOrdering,
+    ubound: BoundOrdering) extends WindowFunctionFrame(ordinal, functions) {
+
+  /** Rows of the partition currently being processed. */
+  private[this] var input: CompactBuffer[InternalRow] = null
+
+  /** Index of the first input row with a value greater than the upper bound of the current
+    * output row. */
+  private[this] var inputHighIndex = 0
+
+  /** Index of the first input row with a value equal to or greater than the lower bound of the
+    * current output row. */
+  private[this] var inputLowIndex = 0
+
+  /** Buffer used for storing prepared input for the window functions. */
+  private[this] val buffer = new util.ArrayDeque[Array[AnyRef]]
+
+  /** Index of the row we are currently writing. */
+  private[this] var outputIndex = 0
+
+  /** Prepare the frame for calculating a new partition. Reset all variables. */
+  override def prepare(rows: CompactBuffer[InternalRow]): Unit = {
+    input = rows
+    inputHighIndex = 0
+    inputLowIndex = 0
+    outputIndex = 0
+    buffer.clear()
+  }
+
+  /** Write the frame columns for the current row to the given target row. */
+  override def write(target: GenericMutableRow): Unit = {
+    var bufferUpdated = outputIndex == 0
+
+    // Add all rows to the buffer for which the input row value is equal to or less than
+    // the output row upper bound.
+    while (inputHighIndex < input.size &&
+        ubound.compare(input, inputHighIndex, outputIndex) <= 0) {
+      buffer.offer(prepare(input(inputHighIndex)))
+      inputHighIndex += 1
+      bufferUpdated = true
+    }
+
+    // Drop all rows from the buffer for which the input row value is smaller than
+    // the output row lower bound.
+    while (inputLowIndex < inputHighIndex &&
+        lbound.compare(input, inputLowIndex, outputIndex) < 0) {
+      buffer.pop()
+      inputLowIndex += 1
+      bufferUpdated = true
+    }
+
+    // Only recalculate and update when the buffer changes.
+    if (bufferUpdated) {
+      evaluatePrepared(buffer.iterator())
+      fill(target, outputIndex)
+    }
+
+    // Move to the next row.
+    outputIndex += 1
+  }
+
+  /** Copy the frame. */
+  override def copy: SlidingWindowFunctionFrame =
+    new SlidingWindowFunctionFrame(ordinal, copyFunctions, lbound, ubound)
+}
+
+/**
+ * The unbounded window frame calculates frames with the following SQL forms:
+ * ... (No Frame Definition)
+ * ... BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
+ *
+ * Its results are  the same for each and every row in the partition. This class can be seen as a
+ * special case of a sliding window, but is optimized for the unbound case.
+ *
+ * @param ordinal of the first column written by this frame.
+ * @param functions to calculate the row values with.
+ */
+private[execution] final class UnboundedWindowFunctionFrame(
+    ordinal: Int,
+    functions: Array[WindowFunction]) extends WindowFunctionFrame(ordinal, functions) {
+
+  /** Index of the row we are currently writing. */
+  private[this] var outputIndex = 0
+
+  /** Prepare the frame for calculating a new partition. Process all rows eagerly. */
+  override def prepare(rows: CompactBuffer[InternalRow]): Unit = {
+    reset()
+    outputIndex = 0
+    val iterator = rows.iterator
+    while (iterator.hasNext) {
+      update(iterator.next())
+    }
+    evaluate()
+  }
+
+  /** Write the frame columns for the current row to the given target row. */
+  override def write(target: GenericMutableRow): Unit = {
+    fill(target, outputIndex)
+    outputIndex += 1
+  }
+
+  /** Copy the frame. */
+  override def copy: UnboundedWindowFunctionFrame =
+    new UnboundedWindowFunctionFrame(ordinal, copyFunctions)
+}
+
+/**
+ * The UnboundPreceding window frame calculates frames with the following SQL form:
+ * ... BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
+ *
+ * There is only an upper bound. Very common use cases are for instance running sums or counts
+ * (row_number). Technically this is a special case of a sliding window. However a sliding window
+ * has to maintain a buffer, and it must do a full evaluation everytime the buffer changes. This
+ * is not the case when there is no lower bound, given the additive nature of most aggregates
+ * streaming updates and partial evaluation suffice and no buffering is needed.
+ *
+ * @param ordinal of the first column written by this frame.
+ * @param functions to calculate the row values with.
+ * @param ubound comparator used to identify the upper bound of an output row.
+ */
+private[execution] final class UnboundedPrecedingWindowFunctionFrame(
+    ordinal: Int,
+    functions: Array[WindowFunction],
+    ubound: BoundOrdering) extends WindowFunctionFrame(ordinal, functions) {
+
+  /** Rows of the partition currently being processed. */
+  private[this] var input: CompactBuffer[InternalRow] = null
+
+  /** Index of the first input row with a value greater than the upper bound of the current
+    * output row. */
+  private[this] var inputIndex = 0
+
+  /** Index of the row we are currently writing. */
+  private[this] var outputIndex = 0
+
+  /** Prepare the frame for calculating a new partition. */
+  override def prepare(rows: CompactBuffer[InternalRow]): Unit = {
+    reset()
+    input = rows
+    inputIndex = 0
+    outputIndex = 0
+  }
+
+  /** Write the frame columns for the current row to the given target row. */
+  override def write(target: GenericMutableRow): Unit = {
+    var bufferUpdated = outputIndex == 0
+
+    // Add all rows to the aggregates for which the input row value is equal to or less than
+    // the output row upper bound.
+    while (inputIndex < input.size && ubound.compare(input, inputIndex, outputIndex) <= 0) {
+      update(input(inputIndex))
+      inputIndex += 1
+      bufferUpdated = true
+    }
+
+    // Only recalculate and update when the buffer changes.
+    if (bufferUpdated) {
+      evaluate()
+      fill(target, outputIndex)
+    }
+
+    // Move to the next row.
+    outputIndex += 1
+  }
+
+  /** Copy the frame. */
+  override def copy: UnboundedPrecedingWindowFunctionFrame =
+    new UnboundedPrecedingWindowFunctionFrame(ordinal, copyFunctions, ubound)
+}
+
+/**
+ * The UnboundFollowing window frame calculates frames with the following SQL form:
+ * ... BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ *
+ * There is only an upper bound. This is a slightly modified version of the sliding window. The
+ * sliding window operator has to check if both upper and the lower bound change when a new row
+ * gets processed, where as the unbounded following only has to check the lower bound.
+ *
+ * This is a very expensive operator to use, O(n * (n - 1) /2), because we need to maintain a
+ * buffer and must do full recalculation after each row. Reverse iteration would be possible, if
+ * the communitativity of the used window functions can be guaranteed.
+ *
+ * @param ordinal of the first column written by this frame.
+ * @param functions to calculate the row values with.
+ * @param lbound comparator used to identify the lower bound of an output row.
+ */
+private[execution] final class UnboundedFollowingWindowFunctionFrame(
+    ordinal: Int,
+    functions: Array[WindowFunction],
+    lbound: BoundOrdering) extends WindowFunctionFrame(ordinal, functions) {
+
+  /** Buffer used for storing prepared input for the window functions. */
+  private[this] var buffer: Array[Array[AnyRef]] = _
+
+  /** Rows of the partition currently being processed. */
+  private[this] var input: CompactBuffer[InternalRow] = null
+
+  /** Index of the first input row with a value equal to or greater than the lower bound of the
+    * current output row. */
+  private[this] var inputIndex = 0
+
+  /** Index of the row we are currently writing. */
+  private[this] var outputIndex = 0
+
+  /** Prepare the frame for calculating a new partition. */
+  override def prepare(rows: CompactBuffer[InternalRow]): Unit = {
+    input = rows
+    inputIndex = 0
+    outputIndex = 0
+    val size = input.size
+    buffer = Array.ofDim(size)
+    var i = 0
+    while (i < size) {
+      buffer(i) = prepare(input(i))
+      i += 1
+    }
+    evaluatePrepared(buffer, 0, buffer.length)
+  }
+
+  /** Write the frame columns for the current row to the given target row. */
+  override def write(target: GenericMutableRow): Unit = {
+    var bufferUpdated = outputIndex == 0
+
+    // Drop all rows from the buffer for which the input row value is smaller than
+    // the output row lower bound.
+    while (inputIndex < input.size && lbound.compare(input, inputIndex, outputIndex) < 0) {
+      inputIndex += 1
+      bufferUpdated = true
+    }
+
+    // Only recalculate and update when the buffer changes.
+    if (bufferUpdated) {
+      evaluatePrepared(buffer, inputIndex, buffer.length)
+      fill(target, outputIndex)
+    }
+
+    // Move to the next row.
+    outputIndex += 1
+  }
+
+  /** Copy the frame. */
+  override def copy: UnboundedFollowingWindowFunctionFrame =
+    new UnboundedFollowingWindowFunctionFrame(ordinal, copyFunctions, lbound)
 }

http://git-wip-us.apache.org/repos/asf/spark/blob/a9a0d0ce/sql/hive/src/test/scala/org/apache/spark/sql/hive/HiveDataFrameWindowSuite.scala
----------------------------------------------------------------------
diff --git a/sql/hive/src/test/scala/org/apache/spark/sql/hive/HiveDataFrameWindowSuite.scala b/sql/hive/src/test/scala/org/apache/spark/sql/hive/HiveDataFrameWindowSuite.scala
index efb3f25..15b5f41 100644
--- a/sql/hive/src/test/scala/org/apache/spark/sql/hive/HiveDataFrameWindowSuite.scala
+++ b/sql/hive/src/test/scala/org/apache/spark/sql/hive/HiveDataFrameWindowSuite.scala
@@ -183,13 +183,13 @@ class HiveDataFrameWindowSuite extends QueryTest {
   }
 
   test("aggregation and range betweens with unbounded") {
-    val df = Seq((1, "1"), (2, "2"), (2, "2"), (2, "2"), (1, "1"), (2, "2")).toDF("key", "value")
+    val df = Seq((5, "1"), (5, "2"), (4, "2"), (6, "2"), (3, "1"), (2, "2")).toDF("key", "value")
     df.registerTempTable("window_table")
     checkAnswer(
       df.select(
         $"key",
         last("value").over(
-          Window.partitionBy($"value").orderBy($"key").rangeBetween(1, Long.MaxValue))
+          Window.partitionBy($"value").orderBy($"key").rangeBetween(-2, -1))
           .equalTo("2")
           .as("last_v"),
         avg("key").over(Window.partitionBy("value").orderBy("key").rangeBetween(Long.MinValue, 1))
@@ -203,7 +203,7 @@ class HiveDataFrameWindowSuite extends QueryTest {
         """SELECT
           | key,
           | last_value(value) OVER
-          |   (PARTITION BY value ORDER BY key RANGE 1 preceding) == "2",
+          |   (PARTITION BY value ORDER BY key RANGE BETWEEN 2 preceding and 1 preceding) == "2",
           | avg(key) OVER
           |   (PARTITION BY value ORDER BY key RANGE BETWEEN unbounded preceding and 1 following),
           | avg(key) OVER

http://git-wip-us.apache.org/repos/asf/spark/blob/a9a0d0ce/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/WindowSuite.scala
----------------------------------------------------------------------
diff --git a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/WindowSuite.scala b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/WindowSuite.scala
new file mode 100644
index 0000000..a089d0d
--- /dev/null
+++ b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/WindowSuite.scala
@@ -0,0 +1,79 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.hive.execution
+
+import org.apache.spark.sql.{Row, QueryTest}
+import org.apache.spark.sql.expressions.Window
+import org.apache.spark.sql.functions._
+import org.apache.spark.sql.hive.test.TestHive.implicits._
+
+/**
+ * Window expressions are tested extensively by the following test suites:
+ * [[org.apache.spark.sql.hive.HiveDataFrameWindowSuite]]
+ * [[org.apache.spark.sql.hive.execution.HiveWindowFunctionQueryWithoutCodeGenSuite]]
+ * [[org.apache.spark.sql.hive.execution.HiveWindowFunctionQueryFileWithoutCodeGenSuite]]
+ * However these suites do not cover all possible (i.e. more exotic) settings. This suite fill
+ * this gap.
+ *
+ * TODO Move this class to the sql/core project when we move to Native Spark UDAFs.
+ */
+class WindowSuite extends QueryTest {
+
+  test("reverse sliding range frame") {
+    val df = Seq(
+      (1, "Thin", "Cell Phone", 6000),
+      (2, "Normal", "Tablet", 1500),
+      (3, "Mini", "Tablet", 5500),
+      (4, "Ultra thin", "Cell Phone", 5500),
+      (5, "Very thin", "Cell Phone", 6000),
+      (6, "Big", "Tablet", 2500),
+      (7, "Bendable", "Cell Phone", 3000),
+      (8, "Foldable", "Cell Phone", 3000),
+      (9, "Pro", "Tablet", 4500),
+      (10, "Pro2", "Tablet", 6500)).
+      toDF("id", "product", "category", "revenue")
+    val window = Window.
+      partitionBy($"category").
+      orderBy($"revenue".desc).
+      rangeBetween(-2000L, 1000L)
+    checkAnswer(
+      df.select(
+        $"id",
+        avg($"revenue").over(window).cast("int")),
+      Row(1, 5833) :: Row(2, 2000) :: Row(3, 5500) ::
+      Row(4, 5833) :: Row(5, 5833) :: Row(6, 2833) ::
+      Row(7, 3000) :: Row(8, 3000) :: Row(9, 5500) ::
+      Row(10, 6000) :: Nil)
+  }
+
+  // This is here to illustrate the fact that reverse order also reverses offsets.
+  test("reverse unbounded range frame") {
+    val df = Seq(1, 2, 4, 3, 2, 1).
+      map(Tuple1.apply).
+      toDF("value")
+    val window = Window.orderBy($"value".desc)
+    checkAnswer(
+      df.select(
+        $"value",
+         sum($"value").over(window.rangeBetween(Long.MinValue, 1)),
+         sum($"value").over(window.rangeBetween(1, Long.MaxValue))),
+      Row(1, 13, null) :: Row(2, 13, 2) :: Row(4, 7, 9) ::
+        Row(3, 11, 6) :: Row(2, 13, 2) :: Row(1, 13, null) :: Nil)
+
+  }
+}


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